The present invention relates to an improved electrostatic chuck or, more particularly, to an improved electrostatic chuck capable of exhibiting a greatly enhanced attractive force to a work piece without a disadvantage of leak currents or dielectric break-down even when the voltage applied to the electrodes is increased.
It is a trend in recent years that the manufacturing process of various kinds of semiconductor devices, liquid crystal display units and the like in the electronic industry involves an increasing number of steps undertaken heretofore in a wet process which, however, are continuously being replaced with steps of a dry process such as the CVD process, sputtering, ion plating and the like in view of the good adaptability to automatization of the process as well as a decrease in the problem of environmental pollution unavoidably caused by the waste liquids used in a wet process. In most of these dry-process treatments of work pieces such as semiconductor silicon wafers, the work piece is worked in a vacuum chamber. In view of the increasing size of the work pieces such as semiconductor silicon wafers, glass plates and the like and the increasing fineness of working on these work pieces as in the patterning works in the manufacture of LSIs to obtain a greatly increased density of circuit integration, it is an essential requirement that the work piece in the vacuum chamber is positioned or transferred with extremely high precision under holding by a precision chucking means.
A traditional chucking means to hold and transfer a work piece is a so-called vacuum chuck in which the work piece is attracted to a chuck head having perforations connected to a suction means such as a vacuum pump to produce a pressure difference between the surfaces of the work piece. As is readily understood from the above mentioned principle, vacuum chucks cannot be used inside of a vacuum chamber because no pressure difference can be produced thereby between the surfaces of a work piece. Although a vacuum chuck can be used under a non-vacuum condition, in addition, vacuum chucks have an inherent problem as a chucking means of a work piece for which extremely high precision is required in working because the attractive force to the work piece is localized at the sucking perforations and not distributed evenly over the whole surface of the chuck head so that a localized stress is caused in the work piece. Accordingly, vacuum chucks cannot be used in a fine working process in the manufacture of electronic devices.
In view of the above described unavoidable problems, vacuum chucks as a means to hold and transfer a work piece are under continuous replacement in recent years with so-called electrostatic chucks, in which the attractive force to the work piece is produced by the static electricity accumulated in the electrodes of the chuck head. By utilizing the feature of electrostatic chucks that the attractive force can be increased as desired by increasing the voltage applied between the electrodes, a study is now under way to use an electrostatic chuck with an object of flatness correction of work pieces such as silicon wafers and glass plates as a substrate of electronic devices, for which an extremely high flatness of the surface is required.
Needless to say, correction of the flatness of a work piece can be effected only by applying a very high attractive force to the work piece. As is known, the electrostatic attractive force f of an electrostatic chuck is given by the equation EQU f=A.multidot..epsilon.(E/t).sup.2,
in which f is the electrostatic force, A is a constant, .epsilon. is the dielectric constant of the insulating material interposed between the work piece and the electrodes, E is the voltage applied between the electrodes and t is the thickness of the insulating layer interposed between the work piece and the electrodes. As is shown by this equation, the electrostatic force f can be increased by increasing the applied voltage E, by decreasing the thickness t of the insulating layer and by using an insulating material having an as high as possible dielectric constant .epsilon.. It is, however, not always possible to satisfy all of these requirements simultaneously. For example, insulating materials having a high dielectric constant generally have a relatively low volume resistivity so that satisfactory insulation can be obtained only by increasing the thickness t of the insulating layer since otherwise a leak current is caused through the work piece or between the fine circuits thereon eventually leading to a damage to the device or circuit under application of a sufficiently high voltage E to cancel the advantage obtained by the use of a highly dielectric material.
A remedial measure in this regard is proposed in Japanese Patent Kokai 62-94953 according to which the insulating material to form the insulating layer is blended with fine particles of a material having a high dielectric constant though with a relatively low volume resistivity such as titanium dioxide, lead titanate and the like dispersed therein. Such a composite insulating material is not always quite satisfactory because, when the content of the highly dielectric material is increased, the withstand voltage of the insulating layer as a whole is greatly decreased so that the device or the circuit on the work piece attracted to the chuck head is sometimes damaged by the electric discharge caused by the dielectric breakdown or leak current on the surface of the chuck head.